Clasification Society Rulefinder 2020 - Version 9.33 - Fix
Common Structural Rules - Common Structural Rules for Bulk Carriers and Oil Tankers, January 2019 - Part 1 General Hull Requirements - Chapter 1 Rule General Principles - Section 2 Rule Principles - 3 Design Basics

3 Design Basics

3.1 General

3.1.1 This sub-section specifies the design parameters and the assumptions about the ship operation that are used as the basis of the design principles of the Rules.

3.1.2 Ships are to be designed to withstand, in the intact condition, the environmental conditions as defined in [5.3.2] and [5.3.3] anticipated during the design life, for the appropriate loading conditions. Structural strength is to be determined against buckling and yielding. Ultimate strength calculations have to include ultimate hull girder capacity and ultimate strength of plates and stiffeners.

3.1.3 Residual strength

Ships having a length L of 150 m or above are to be designed to have sufficient reserve strength to withstand the loads in damaged conditions, e.g. collision, grounding or flooded scenarios. Residual strength calculations are to take into account the ultimate reserve capacity of the hull girder, considering permanent deformation and post-buckling behaviour as specified in Ch 5, Sec 3.

3.1.4 Finite element analysis

The scantling of the structural members within the cargo hold region of ships having a length L of 150 m or above is to be assessed according to the requirements specified in Pt 1, Ch 7.

3.1.5 Fatigue life

Ships having a length L of 150 m or above are to be assessed according to the design fatigue life for structural details specified in Pt 1, Ch 9.

3.1.6 The Rules are applicable for ships in compliance with the specified design basis. Special consideration is given to deviations from this design basis.

3.1.7 The design basis used for the design of each ship, as communicated by the builder to the owner, is to be documented and submitted to the Society as part of the design review and approval. All changes of the design basis are to be formally advised to the Society and the owner for approval.

3.2 Hull form limit

3.2.1 The Rules assume the following hull form with respect to environmental loading:

  • L < 500 m
  • CB > 0.6
  • L/B > 5
  • B/D < 2.5

For ships over 350 m in length, special consideration is to be made for the wave loads by the Society.

3.3 Design life

3.3.1 A design life of 25 years is assumed for selecting ship design parameters. The specified design life is the nominal period that the ship is assumed to be exposed to operating conditions.

3.4 Environmental conditions

3.4.1 North Atlantic wave environment

The rule requirements are based on a ship trading in the North Atlantic wave environment for its entire design life.

3.4.2 Wind and current

The effects of wind and current with regard to the strength of the structure are not considered.

3.4.3 Ice

The effects of ice and ice accretion are not taken into account by the Rules.

3.4.4 Design temperatures

The Rules assume that the structural assessment of hull strength members is valid for the following design temperatures:

  • Lowest mean daily average temperature in air is -10°C.
  • Lowest mean daily average temperature in seawater is 0°C.

Ships intended to operate in areas with lower mean daily average temperature, e.g. regular service during winter seasons to Arctic or Antarctic waters are subject to the requirements as specified by the Society.

In the above, the following definitions apply:
  • Mean : Statistical mean over observation period (at least 20 years).
  • Daily Average : Average during one day and night.
  • Lowest : Lowest during year.

For seasonally restricted service the lowest value within the period of operation applies.

3.4.5 Thermal loads

The effects of thermal loads and residual stresses are not taken into account in the Rules.

3.5 Operating conditions

3.5.1 The Rules specify minimum loading conditions that are to be assessed for compliance.

Specification of loading conditions other than those required by the Rules is the responsibility of the owner. These other loading conditions are to be documented and also be assessed for compliance.

3.5.2 The Rules assume the following:
  • The ballast cargo hold of bulk carriers is not to be partly filled in seagoing operations.
  • Ballasting and deballasting operations in the ballast cargo hold of bulk carriers are not to be performed when the weather is not fair.

3.6 Operating draughts

3.6.1 The design operating draughts are to be specified by the builder/designer subject to acceptance by the owner and are to be used to derive the appropriate structural scantlings. All operational loading conditions in the loading manual are to comply with the specified design operating draughts. The following design operating draughts are as a minimum to be considered:
  • Scantling draught for the assessment of structure.
  • Minimum ballast draught at midship for assessment of structure.
  • Minimum heavy ballast draught at midship for assessment of bulk carrier structure.
  • Minimum forward draughts for the assessment of bottom structure forward subjected to slamming loads, TF-e and TF-f, with and without ballast tanks in way filled.

    TF-e and TF-f are defined in Ch 4, Sec 5, [3.2.1]

  • For oil tankers: maximum draughts amidships for both conditions:
    • with all cargo tanks abreast empty.
    • with centre cargo tank empty and wing cargo tanks full.
    • with centre cargo tank full and wing cargo tanks empty.
  • For bulk carriers carrying steel coils: maximum draught amidships for steel coil loading conditions.

3.7 Internal environment

3.7.1 Oil cargo density for strength assessment

A density of 1.025 t/m3, or a higher value if specified by the designer, is to be used for oil cargoes for the strength assessment of all relevant tank structures.

3.7.2 Oil cargo density for fatigue assessment

For the fatigue assessment of cargo tank structures, the mean density is to be taken as 0.9 t/m3, or a higher value if specified by the designer.

3.7.3 Dry cargo density

The density for dry bulk cargo is to be taken according to the specifications in Ch 4, Sec 6, [2.3].

3.7.4 Water ballast density

A density of 1.025 t/m3 is to be used for water ballast.

3.8 Structural construction and inspection

3.8.1 The structural requirements included in the Rules are developed with the assumption that construction and repair follow acceptable shipbuilding and repair standards and tolerances. The Society may require that additional attention is paid to critical areas of the structure by the builder during construction and by the owner for repair after the ship’s delivery.

3.8.2 As an objective, ships are to be built in accordance with controlled quality production standards using approved materials as necessary.

3.8.3 The Rules define the renewal criteria for the individual structural items. The structural requirements included are developed on the assumption that the structure is subject to appropriate monitoring by the owner once the ship is in operation and to periodical survey in accordance with Society rules and regulations.

3.8.4 Tank strength and tightness testing are to be carried out as a part of the verification scheme according to the Rules and/or documents of the individual Society which incorporate IACS UR S14.

3.8.5 Specifications for material manufacturing, assembling, joining and welding procedures, steel surface preparation and coating are to be included in the ship construction quality procedures. It is assumed that the owner has approved these builder specifications.

3.9 Maximum service speed

3.9.1 The maximum service speed is to be specified in the design specification. Although the hull structure verification criteria takes into account the service speed this does not relieve the responsibilities of the owner and personnel to properly handle the ship, see item (d) in [2.2.2].

3.10 Owner’s extras

3.10.1 Owner’s specification of requirements above the general classification or statutory requirements may affect the structural design. Owner’s extras may include requirements for:
  • Vibration analysis.
  • Maximum percentage of high strength steel.
  • Additional scantlings above that required by the Rules.
  • Additional design margin on the loads specified by the Rules, etc.
  • Improved fatigue resistance, in the form of a specified increase in design fatigue life or equivalent.

Owner’s extras are not specified by these Rules. Owner’s extras, if any, that may affect the structural design are to be clearly specified in the design documentation.

Parent topic: Section 2 Rule Principles
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